1. Field of the Invention
The present invention relates to an electroluminescence display device provided with an electroluminescence element and a thin film transistor.
2. Description of the Related Art
In recent years, electroluminescence (hereinafter referred to as EL) display devices employing EL elements have attracted attention as an alternatives to devices such as CRTs and LCDs. For example, EL display devices including a thin film transistor (hereinafter referred to as TFT) used as a switching element for driving the EL elements have been researched and developed.
FIG. 1 is an equivalent circuit diagram of a related art EL display device including an EL element and TFTS.
The EL display device shown in FIG. 1, which illustrates a portion near a gate signal line Gn in the nth row and a drain signal line Dm in the mth column, includes first and second TFTS 130 and 140, and an organic EL element 160.
The gate signal line Gn for supplying a gate signal and a drain signal line Dm for supplying a drain signal cross each other, and the organic EL element 160 and the TFTs 130 and 140 for driving the organic EL element 160 are provided near an intersection of these signal lines.
The first TFT 130 for switching operation includes a gate electrode 131 connected to the gate signal line Gn and receiving the gate signal, a drain electrode 132 connected to the drain signal line Dm and receiving the drain signal, and a source electrode 133 connected to a gate electrode 141 of the second TFT 140.
The second TFT 140 for driving the organic EL element includes the gate electrode 141 connected to the source electrode 133 of the first TFT 130, a source electrode 142 connected to an anode 161 of the organic EL element 160, and a drain electrode 143 connected to a driving power supply 150 for supplying a current to the organic EL element 160.
The organic EL element 160 includes the anode 161 connected to the source electrode 142, a cathode 162 connected to a common electrode 164, and a light emissive element layer 163 sandwiched between the anode 161 and the cathode 162.
The device further includes a storage capacitor 170 having one electrode 171 connected between the source electrode 133 of the first TFT 130 and the gate electrode 141 of the second TFT 140, and the other electrode 172 connected to a common electrode 173.
A method of driving the circuit shown in the equivalent circuit diagram of FIG. 1 will next be described with reference to signal timing charts in FIG. 2, in which signal timings of a signal VG(n)1 supplied to the gate electrode 131 of the first TFT 130 in the nth row, a drain signal VD at the drain signal line Dm, and a signal VG(n)2 supplied to the gate electrode 141 of the second TFT 140 in the nth row are illustrated in (a)-(c), respectively.
When the gate signal VG(n)1 illustrated in FIG. 2(a) is applied from the gate signal line Gn to the gate electrode 131, the first TFT 130 is switched on. As a result, the drain signal VD illustrated in FIG. 2(b) is supplied from the drain signal line Dm to the gate electrode 141, which attains the same potential as the drain signal line Dm. A current corresponding to the value of a voltage applied to the gate electrode 141 is then supplied from the driving power supply 150 to the EL element 160, which is caused to emit light.
It should be noted that in actual operation, while the first TFT 130 is in ON state, a current flows until the gate electrode 141 attains the same potential as the drain signal line Dm and electric charges are stored in a gate capacitor of the gate electrode 141. After the first TFT 130 is turned off,the electric charges stored in the gate capacitor must be maintained, and also the potential of the gate must be retained as illustrated by the broken line in FIG. 2(c).
In the above-described EL display device, however, a leakage current flows during the OFF period of the TFT. As a result, when the drain signal VD changes every horizontal period (IH) as illustrated in FIG. 2(b), the potential VG(n)2 of the gate electrode 141 cannot be maintained, but is changed as shown by the solid line in FIG. 2(c).
More specifically, as illustrated by the solid line in FIG. 2(c), (i) when the potential of the drain signal line Dm is lower than that supplied to the gate electrode 141, a leakage current flows to the drain signal line Di through the first TFT 130, decreasing the potential of the gate electrode 141; and (ii) when the potential of the drain signal line Dm is higher than that supplied to the gate electrode 141, a leakage current flows to the gate electrode 141 through the first TFT 130, resulting in further storage of electric charges in the gate capacitor and in a higher potential of the gate electrode 141.
In configuration (i) above, the organic EL element 160 receives a current larger than it is supposed to receive, leading to a higher luminance of the organic EL element. On the other hand, the element 160 will have a lower luminance in configuration (ii) above.
With either configuration, the device has a drawback in that it is difficult to cause each display pixel to emit light at the appropriate luminance when a large leakage current flows through the first TFT 130 as indicated by the solid line in FIG. 2(c).
The second TFT serves to control the current supplied from the power supply for driving the organic EL element in accordance with the voltage applied to the gate of the second TFT, and supply it to the organic EL element. The second TFT has an active layer that includes an intrinsic or substantially intrinsic channel region overlapping its gate, and source and drain regions located on both sides of the channel region and having impurities doped therein.
However, when the second TFT is an n-channel transistor, a so-called saturation region of the drain current-drain voltage (Id-vd) characteristics, i.e. a region where the drain current Id is constant even though the drain voltage vd is increased, is extremely narrow (saturation characteristics are poor), as indicated by broken lines in FIG. 3B. Consequently, the current value Id is increased with an increase in the value of Vd, and therefore a constant current cannot be obtained, but is affected by the voltage Vd, leading to a poor current controllability.
Especially when the TFT is formed of polycrystalline silicon, there exist grain boundaries of crystals, and electrons are trapped therein to form a potential barrier, thereby spreading a depletion layer. As a result, a strong electric field is applied to the grain boundaries at the edges of the drain electrode, whereby a collisional ionization phenomenon, in which accelerated electrons collide with lattices, occurs and the drain current is not saturated, but increased. This is a significant problem in the n-channel TFT, but rare in the p-channel TFT.
The present invention has been conceived in view of the above-described problems, and an object thereof is to provide an EL display device which creates a superior gradation (gray scale) display by, expressed in the terminology introduced above, suppressing a leakage current at the first TFT 130 to maintain the potential at the gate electrode 141 of the second TFT 140, and improving current controllability of the second TFT 140.
According to one aspect, the present invention provides an electroluminescence device which includes an electroluminescence element having a light emissive layer provided between first and second electrodes, a first thin film transistor receiving a selection signal at its gate to acquire a data signal, and a second thin film transistor provided between a driving power supply and the electroluminescence element and controlling power supplied from the driving power supply to the electroluminescence element in accordance with the data signal applied from the first thin film transistor. In this electroluminescence device, the first thin film transistor has an n-channel, and at least one of a lightly doped drain structure, an offset structure, and a multigate structure, and the second thin film transistor has a p-channel.
By employing an n-channel transistor as the first thin film transistor in this manner, a quick response can be achieved at the transistor. Further, by employing at least one of the lightly doped drain structure, the offset structure, and the multigate structure in the first thin film transistor, an off-leakage current at the transistor can be reduced, and a quick response can be made in accordance with the selection signal applied to the gate to acquire and surely maintain the data signal. When a p-channel transistor is used for the second thin film transistor, a stable current can be output in response to the drain voltage, making it possible to supply power (or current when an organic EL element is used) to the electroluminescence element from the driving power supply in a stable manner, and to suppress variation in emitted luminance at the element.
According to another aspect of the present invention, the first and second thin film transistors of the electroluminescence device include an active layer formed of non-single crystalline semiconductor layer.
According to a still another aspect of the present invention, the non-single crystalline semiconductor layer is a polycrystalline silicon layer.
According to a further aspect of the present invention, the first and second thin film transistors are of top gate or bottom gate type, having a gate electrode above or below their active layer.
According to a further aspect of the present invention, the electroluminescence element and the first and second thin film transistors form a pixel of the device, and a plurality of such pixels are arranged in a matrix on a substrate.
According to a further aspect of the present invention, drains of the respective first thin film transistors for the pixels assigned to the same column among the plurality of pixels arranged in the matrix are connected to the same data line, while gates of the respective first thin film transistors for the pixels assigned to the same row among the plurality of pixels arranged in the matrix are connected to the same scan line.
According to a further aspect of the present invention, the electroluminescence device further includes a storage capacitor connected between a source of the first thin film transistor and a gate of the second thin film transistor, and the non-single crystalline semiconductor layer used for the active layer of the first thin film transistor also serves as one electrode of the storage capacitor.
By thus providing a storage capacitor for each pixel, the data signal acquired while the first thin film transistor is selected and thus in the ON state can be maintained until the first thin film transistor is next selected, contributing to improvement in display quality. In addition, by thus forming one electrode of the storage capacitor by the non-single crystalline semiconductor film used as the active layer of the transistor, increase in number of manufacturing steps for forming the electrodes of the storage capacitor can be minimized.
According to a further aspect of the present invention, the electroluminescence element is an organic electroluminescence element having a light emissive layer formed of an organic compound. In a color display device or the like, it can be extremely advantageous to have such a light emissive layer formed of an organic compound because this configuration provides a wide variety of displayable colors and a wide range of options for the material used.
According to a further aspect, the present invention provides an electroluminescence display device which includes an electroluminescence element having a light emissive layer provided between an anode and a cathode; a first thin film transistor having an active layer which is formed of a non-single crystalline semiconductor film and which includes a source connected to a storage capacitor, a drain connected to a drain signal line, and a gate electrode provided over a channel of the active layer and connected to a gate signal line; and a second thin film transistor having an active layer which is formed of a non-single crystalline semiconductor film and which includes a drain connected to a driving power supply of the electroluminescence element, and a gate electrode connected to the source of the first thin film transistor. In the above electroluminescence display device, the first thin film transistor has an n-channel and at least one of a lightly doped drain structure, an offset structure, and a multigate structure, while the second thin film transistor has a p-channel.
In a further aspect, the present invention provides an electroluminescence display device which includes an electroluminescence element having a light emissive layer provided between an anode and a cathode; a first thin film transistor having an active layer which is formed of a non-single crystalline semiconductor film and which includes a source connected to a storage capacitor, a drain connected to a drain signal line, and a gate electrode provided under a channel of the active layer and connected to a gate signal line; and a second thin film transistor having an active layer which is formed of a non-single crystalline semiconductor film and which includes a drain connected to a driving power supply of the electroluminescence element, and a gate electrode connected to the source of the first thin film transistor. In the above electroluminescence display device, the first thin film transistor has an n-channel and at least one of a lightly doped drain structure, an offset structure, and a multigate structure, while the second thin film transistor has a p-channel.
Because the above-described EL device and EL display device according to the present invention includes the first TFT allowing high speed writing with good retention characteristics and the second TFT with good current controllability, an excellent gradation (gray scale) display can be realized.